1. Field of the Invention
The present invention is directed to a method and apparatus for generating a print image, particularly a franking imprint, on an image carrier, of the type wherein the print image is composed of at least one first partial image and a second partial image that are transversely offset relative to a direction, wherein a relative motion between a print head and the image carrier is generated along a first direction in a first step for generating the first partial image, a transverse offset between the print head and the image carrier is generated in a second direction proceeding transverse to the first direction in a second step, and a relative motion between the print head and the image carrier is generated along the first direction in a third step for generating the second partial image.
2. Description of the Prior Art
A method and apparatus of the above type are disclosed, for example, in European Application 0 980 762, wherein the print head generates a franking imprint on a letter in two partial images offset transversely relative to one another, those partial images supplementing one another to form a complete franking imprint on the letter. The print head is first moved across the letter in a first direction and thereby generates the first partial image. Subsequently, it is offset transversely relative to this first direction by an offset device in the second set before it is moved over the letter in a direction opposite the first direction, whereby the second partial image is then generated on the letter.
It is especially important in postage meter machines, wherein a monetary imprint, namely for franking a letter or the like, is generated to reduce the possibilities for manipulation with fraudulent intent (temporary) to a minimum, or to even preclude tampering entirely.
One possibility for tampering in conjunction with the aforementioned known postage meter machine is to obtain access to the control data for the drive of the nozzles of the print head by fraudulent manipulations at the postage meter machine and to forward in parallel to a number of printer devices (referred to as a parallel attack) in order to simultaneously generate one and the same franking imprint at the different machines.
This particular security risk is significant for postage meter machines because the internal postage calculation ensues based on the generation of the control data, and, moreover, the region of the print head at which such sensitive data must ultimately arrive usually must be accessible at least for replacement, so that manipulations are easier to carry out at that location.
In known postage meter machines, tampering protection usually takes the form of making access to the control data more difficult, usually with mechanical impediments such as diaphragms, covers, etc. This, however, has the disadvantage that the design of the print head and the leads to the print head usually have a relatively complicated form as a result.